Meals in schools and hospitals will be tested for horsemeat as part of a nationwide probe into processed beef products, the Environment Secretary has told Sky News.

Speaking ahead of an emergency summit with food producers, supermarkets and health officials, Owen Paterson, when asked if it was likely that school and hospital meals contained horsemeat, said: "We will be testing those as well, alongside products in retailers."

The Government has been unable to guarantee that state-provided food does not contain horsemeat.

The talks came as the retailer Aldi confirmed some of its ready meals contained 100% horsemeat, while Findus has admitted that it knew its products were affected more than a week ago.

He said he was "determined to get to the bottom" of the matter and to see if "it is gross incompetence or a serious criminal conspiracy".

The Environment Secretary added that "retailers are ultimately responsible for what they sell".

The Food Standards Agency (FSA) also attended the meeting on Saturday. It has ordered all British retailers and processors to test all their processed beef products to make sure they are what they say they are within a week.

Mr Paterson said they would examine how the existing system works and how horsemeat got into the food network at the summit.

Shadow environment secretary Mary Creagh told Sky News Labour hoped the meeting would produce "some clarity from Government about what consumers should be doing and whether the Government's testing regime was going to work".

She added that, according to the food industry, the testing regime put in place by ministers was "completely unworkable".

With only six laboratories in the UK that can do the necessary DNA tests, testing hundreds of product lines in every supermarket would not be possible, she warned.

Meanwhile, the Aldi supermarket chain has confirmed that two of its ready meal ranges produced by Comigel, the French supplier also used by Findus, were found to contain between 30% and 100% horse meat.

The dishes affected are Today's Special frozen beef lasagne and Today's Special frozen spaghetti Bolognese.

The company said it felt "angry and let down" by Comigel and that anyone who had bought the affected products was entitled to a full refund.

Comigel, the company at the centre of the latest scare, whose headquarters are based in Luxembourg, has told the AFP news agency that the horsemeat originated in a Romanian abattoir.

The meat was supplied to Comigel by a meat-processing company called Spanghero, based in southwestern France.

The scandal has spread across continental Europe, with Findus withdrawing various frozen meals from both France and Sweden.

Swedish firm Findus has also pledged to sue an unidentified party over the matter, saying it was "deceived".

Speaking from Luxembourg, Sky News Home Affairs Correspondent Mark White said authorities now faced a "complex and very difficult investigative process" to trace the contaminated meat.

He said authorities had acknowledged that the meat may have come from more than one source.

Video:Horsemeat Scandal Raises Questions

"The French and the Luxembourg authorities say they have traced the contaminated meat to a supplier in France, but it's not as simple as that.

"They say there are multiple other suppliers into this French company and tracing them back to the source is proving much more problematic."

Politicians and food safety experts have played down the risk to human health, with Prime Minister David Cameron saying it was not about food safety, but about accurate food labelling.

He said the industry would have to work hard to restore consumer confidence.

Meanwhile, Labour MP Tom Watson has published a letter on his website which he claims was sent by Findus to retailers on Monday, warning that a France-based supplier had told it there may be problems with raw materials delivered since August 1 last year.

In it, the firm claimed raw materials delivered to a French contractor since August 1 were "likely to be non-conform and consequently the labelling on finished products is incorrect".

The letter added: "The supplier has asked us to withdraw the raw material batches."

Responding to the claims, Findus said they did not know about problems back in August and that they first suspected an issue on January 22, when they ordered the initial tests.

The product recall was ordered on February 2 after further tests had been conducted.

In a statement the company said they were only made aware of a possible August 2012 date through a letter dated February 2 this year, by which time it was "already conducting a full supply chain traceability review and had pro-actively initiated DNA testing".

Findus said it had not been invited to the Government summit but they were aware that the Food and Drink Federation, of which they are a member, was attending.